Biosimilar Rituximab in <scp>ANCA</scp> ‐Associated Vasculitis Compared to the Originator: A Multicenter Cohort Study

OBJECTIVE: To evaluate the six-month effectiveness and safety of rituximab biosimilars compared to the originator in granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA), and outcomes following originator to biosimilar switching. METHODS: We recruited adults with GPA or MPA treated with the rituximab originator or a biosimilar for induction or maintenance, or who switched from originator to biosimilar maintenance (2018-2023). Participants either started the treatment within the preceding six months or were recruited from vasculitis research cohorts. Outcomes included six-month remission (Birmingham Vasculitis Activity Score [BVAS] version 3 of 0), relapse (BVAS rise after achieving remission requiring treatment), vasculitis damage, and serious adverse events (SAEs). RESULTS: We studied 207 participants from 9 centers: 132 starting induction (58 originator and 74 biosimilar), 59 starting maintenance (23 originator and 36 biosimilar), and 16 in the "switch" group. Mean age was 56.7 years (SD 17.5), 52% were female, 80% were White, and 70% had GPA. At six months from induction, 51 of 53 (96%) originator and 66 of 73 (90%) biosimilar recipients achieved remission (difference 6%, 95% confidence interval [CI] -4% to 15%), whereas at three months (exploratory) 48 of 51 (94%) and 57 of 72 (79%) recipients, respectively, had achieved remission (difference 15%, 95% CI 2% to 26%). All individuals in the maintenance and "switch" subgroups were in remission at six months. One minor relapse occurred in each of the induction groups and in the originator maintenance group. Change in vasculitis damage and SAEs were not significantly different between groups. CONCLUSION: This study found no significant differences in six-month outcomes between the rituximab originator and biosimilars for induction of GPA and MPA, with no concerning early signals in those initiating maintenance or switching from the originator to a biosimilar.